Micro purge of plasma region

ABSTRACT

An analysis system includes a laser source generating a laser beam for creating a plasma at a location on a sample. A spectrometer is responsive to photons emitted by the sample at said location and has an output. At least one nozzle is configured to deliver inert gas from a source locally to the location on the sample. A controller is responsive to a trigger signal and is configured to activate the laser source generating a series of laser pulses, open a valve to purge the location locally on the sample, and close the valve after one or more laser pulses.

FIELD OF THE INVENTION

The subject invention relates to spectroscopic instruments.

BACKGROUND OF THE INVENTION

Various spectroscopic instruments are known. X-ray based instruments,for example, can be used to determine the elemental make up of a sampleusing x-ray florescence spectroscopy. Portable XRF has become apreferred technique for elemental analysis in the field. Portable XRF isfast, non-destructive, and provides reasonably accurate results (i.e.,quantification of elemental concentrations in a wide variety ofsamples). With XRF, an x-ray tube is used to direct x-rays at a sample.Atoms in the sample absorb x-rays and re-emit x-rays that are unique tothe atomic structure of a given element. A detector measures the energyof each x-ray and counts the total number of x-rays produced at a givenenergy. From this information, the types of elements and theconcentration of each element can be deduced. Commercially availableanalyzers include the Delta manufactured by Olympus NDT and the NitonXLT-3 manufactured by Thermo Fisher Scientific.

X-rays, however, pose a safety concern. Also, portable and benchtop XRFanalyzers have not to date been used to determine lower atomic numberelements such as beryllium, sodium, carbon, boron, oxygen, nitrogen,lithium, and the like.

Laser induced break down spectroscopy (LIBS) devices are known and usedto detect the elemental concentration of lower atomic numbered elementswith some accuracy. These devices typically include a high powered laserthat sufficiently heats a portion of the sample to produce a plasma. Asthe plasma cools, eventually the electrons return to their groundstates. In the process, photons are emitted at wavelengths unique to thespecific elements comprising the sample. The photon detection andsubsequent measurement of elemental concentrations are similar to sparkoptical emission spectroscopy (OES). Examples of LIBS devices are theLIBS SCAN 25 from Applied Photonics, the LIBS25000 from Ocean Optics,and the RT 100 from Applied Spectra.

Some elements such as carbon, phosphorous, and sulfur react with oxygenresulting in a very low level signal which can be difficult to detectand/or properly analyze.

It is known to use an inert gas such as argon to purge the sample.Typically, the flow rate is high and the area purged is large. The gasmay be used to purge a sample chamber in some prior art LIBS analysissystems. Accordingly, a large source (e.g., a tank) of argon gas isrequired and must be toted along in the field. Other analysis systemsusing an argon purge, such as a mobile spark OES analyzer, also usequite a lot of argon gas for purging.

SUMMARY OF THE INVENTION

In a LIBS device, it is desirable to use eye-safe lasers. One example ofan eye-safe laser with enough power for LIBS usage is one at 1.5 micronwavelength. Other wavelengths are possible. Water absorbs heavily atthis wavelength thus preventing the laser reaching the retina of theeye. Devices with eye-safe lasers receive a regulatory rating of eitherClass 1 or Class 2 depending upon the power level of the laser. Class 1is the most desired because it is the least regulated. For handhelddevices which operate in an open beam configuration, the Class 1 orClass 2 rating is highly desired because it yields the maximum operatorsafety and is subject to the least amount of regulation.

Because of the lower pulse energies currently available from 1.5 μmlasers, it is often necessary to focus the laser into a smaller spotsize, typically 100 μm or less in order to get a high enough powerdensity to ignite a plasma. Lower power lasers than are commonly usedfor bench top LIBS instruments are also desirable particularly in thecase of a handheld or portable LIBS unit due to size and powerrestrictions imposed to maintain portability of the instrument. The verysmall beam spot size on the sample creates three problems that should besolved to make a LIBS device commercially viable. First, the laser mustbe focused precisely on the surface of the sample being analyzed forconsistent analytical results. Second, the sample must be clean fromsurface contamination including oxidation on the same distance scale of100 μm or less. Third, some samples are non-homogeneous. Thus, on asample, locations even a small distance away from each other my yielddifferent elements and/or different elemental concentrations. It istherefore desirable to design such a LIBS device to make severalmeasurements at different regions of the sample and combine the results.The invention disclosed includes an eye-safe laser in one preferredembodiment. However, the invention is useful for lasers of otherwavelengths and/or larger beam spots on the sample.

In one preferred example, a spectrometer system, preferably handheld orotherwise portable, is provided and is configured to automatically,based on spectral information, properly focus the laser on the sample,clean the sample, and analyze different locations on the sample.

In a portable, battery powered device, it is not desirable to requirethe user to carry a large tank of purge gas. In one preferredembodiment, a purge subsystem allows a small argon cartridge to be used(e.g., 3-6″ long) because the purge gas is conserved. The flow rateduring testing is low and the gas flow is directed only locally to thelocation on the sample where the plasma is generated by the laser beam.Moreover, the purge gas is supplied only just before testing and turnedoff at the end of a test (or even before). In this way, the purge gas isfurther conserved.

Featured is an analysis system comprising a laser source generating alaser beam for creating a plasma at a location on a sample, aspectrometer responsive to photons emitted by the sample at the locationand having an output, and at least one nozzle configured to deliverinert gas from a source locally to the location on the sample. A valveis located between a source of inert gas and the nozzle. A triggermechanism generates a trigger signal. A controller is responsive to thetrigger signal and is configured to activate the laser source generatinga series of laser pulses, to open the valve to purge the locationlocally on the sample, and to close the valve after one or more laserpulses.

The controller may be configured to process the spectrometer outputbetween pulses. In some examples, the nozzle is positioned andconfigured to produce an inert gas purge spray having a volume less than1.0 cm³. The nozzle is preferably configured to produce an inert gasspray at a flow rate of less than 2 CFH. The controller is preferablyconfigured to open the valve to purge the location locally on the samplebefore and proximate the first laser pulse.

The source of inert gas may be a cartridge. In some embodiments, theanalysis system is a portable, battery powered unit and the gascartridge is loadable within the unit.

A battery powered, portable LIBS analyzer in accordance with examples ofthe invention include a laser source generating a laser beam forcreating a plasma at a location on a sample, a spectrometer responsiveto photons emitted by the sample at the location and having an output,cartridge of inert gas, and a nozzle configured to deliver inert gasfrom the cartridge locally to the location on the sample in a volumeless than 1.0 cm³ and at a flow rate of less than 2 CFH. A valve isdisposed between the cartridge of inert gas and the nozzle. A triggermechanism generates a trigger signal. A controller is responsive to thetrigger signal and is configured to activate the laser source generatinga series of laser pulses, to open the valve to purge said locationlocally on the sample prior to and proximate a first laser pulse, and toclose the valve after one or more laser pulses. The cartridge may beloadable with the analyzer.

One analysis method includes activating a laser source to generate aseries of laser pulses striking a sample at a location, directing aninert gas at said location, and delivering said inert gas to saidlocation at a predetermined flow rate and at a predetermined volumeprior to and proximate the first laser pulse. In one example, thepredetermined flow rate is less than 2 CFH, the predetermined volume isless than 1.0 cm³, and the inert gas is delivered less than 0.5 secondsbefore the first laser pulse.

Also featured is a method making an analysis system. A laser sourcegenerates a laser beam for creating a plasma at a location on a sample.The method includes configuring at least one spectrometer to beresponsive to photons emitted by the sample at said location, includingat least one nozzle to deliver inert gas from a source locally to saidlocation on the sample, and incorporating a valve between a source ofinert gas and the nozzle. The method also includes programming acontroller to be responsive to a trigger signal and to activate thelaser source generating a series of laser pulses, open the valve topurge said location locally on the sample, and close the valve after oneor more laser pulses.

The controller can be further configured to process a spectrometeroutput between pulses. Preferably, the nozzle is positioned andconfigured to produce an inert gas purge spray having a volume less than1.0 cm³, to produce an inert gas spray at a flow rate of less than 2CFH.

Featured is an analysis system comprising a moveable focusing lens, alaser having an output directed at the focusing lens, a spectrometeroutputting intensity data from a sample. A controller system isresponsive to the spectrometer and is configured to energize the laser,process the output of the spectrometer, and adjust the position of thefocusing lens relative to the sample until the spectrometer outputindicates a maximum or near maximum intensity resulting from a laseroutput focused to a spot on the sample. In this manner, an eye safelaser may be used.

In some embodiments, the detection path is through the focusing lens tothe spectrometer. The laser output wavelength may be approximately 1.5μm. The laser may be as low as class 1 laser with a focused spot sizeequal to or less than 100 μm on the sample.

The intensity is preferably an integrated intensity over a plurality ofwavelengths. Adjusting the position of the focusing lens may includemoving it away from the sample and towards the sample.

Also featured in some example is a moveable optic configured to directfocused laser energy to multiple locations on the sample. The controllersystem may further be configured to initiate a moving spot cycle whereinthe orientation of the moveable optic is adjusted and again the laser isenergized and the output of the spectrometer processed. The controllersystem may be configured to terminate the moving spot cycle when thespectrometer output does not change by a predetermined amount betweendifferent sample locations. Preferably, the controller system isconfigured to adjust the position of the focusing lens at each samplelocation. In one example, the movable optic includes the focusing lens.In other examples, the movable optic includes one or more mirrors or aglass optic.

In some examples, the controller system may be configured to initiate acleaning cycle and to terminate the cleaning cycle—processing thespectrometer output and energizing the laser in a cleaning mode untilthe output stabilizes. The cleaning cycle may automatically terminatewhen a rolling average of at least one peak intensity changes by lessthan a predetermined percentage. The controller can be configured tomove the position of the focusing lens producing a larger spot sizeduring the cleaning cycle and to return the focusing lens to a focusedposition after terminating the cleaning cycle

Also featured is an analysis system comprising an adjustable focusinglens, a laser having an output directed at the focusing lens, a moveablecomponent configured to direct laser energy to multiple locations on asample, and a spectrometer outputting intensity data from the sample. Acontroller system is responsive to the spectrometer and is configured toinitiate a focusing cycle wherein the laser is energized, thespectrometer output is analyzed, and the position of the focusing lensis adjusted until the spectrometer output is optimized resulting from alaser output focused on the sample. The system initiates a cleaningcycle wherein the laser is energized, the spectrometer output isanalyzed, and the cleaning cycle terminates when the spectrometer outputstabilizes. The system initiates a moving spot cycle wherein the movablecomponent is adjusted and the spectrometer output is analyzed formultiple locations on the sample.

Also featured is an analysis method comprising energizing a laserproducing a beam impinging on a sample, analyzing the resulting photons,and based on the analysis, automatically adjusting the focus of thelaser beam on the sample to produce a focused spot on the sample. Thefocus of the laser beam may be adjusted until a maximum or near maximumintensity is reached at one or more wavelengths. Photons may be directedfrom the sample along a detection path through the focusing lens to adetector system.

The method may further include cleaning the sample using the laser beam.Cleaning can include adjusting the focus of the laser to produce alarger spot. Cleaning the sample may include energizing the laser,analyzing the resulting photons, and terminating cleaning when anintensity stabilizes.

One method may include moving the beam to multiple locations on thesample and optionally adjusting the focus of the laser beam at eachlocation. The method may include cleaning the sample at each locationusing the beam. The beam can be moved until analysis of the sampleindicates a homogeneous sample. For a non-homogeneous sample, the beammay be moved a predetermined maximum number of times.

Also featured is a spectroanalysis method comprising directing a laseroutput at an adjustable focusing lens, detecting intensity data from thesample, and initiating a focusing cycle wherein the laser is energized,the intensity data is analyzed, and the position of the focusing lens isadjusted until the intensity data is optimized resulting in a laseroutput focused on the sample. A cleaning cycle is initiated wherein thelaser is energized, the intensity data is analyzed, and the cleaningcycle terminates when the intensity data stabilizes. A moving spot cycleis initiated wherein the laser output is moved to a new location on thesample and the intensity data is analyzed for multiple locations on thesample.

In some examples, the focusing lens may be adjusted to make thepredetermined spot larger during the cleaning cycle. The focusing cycleand cleaning cycle may be initiated for each location on the sampleduring the moving spot cycle. In one example, the moving spot cycleterminates when the intensity data indicates the sample is homogeneous.

The subject invention, however, in other embodiments, need not achieveall these objectives and the claims hereof should not be limited tostructures or methods capable of achieving these objectives.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Other objects, features and advantages will occur to those skilled inthe art from the following description of a preferred embodiment and theaccompanying drawings, in which:

FIG. 1 is a block diagram showing an example of a spectrometer system inaccordance with the invention;

FIG. 2 is a block diagram showing another example of a spectrometersystem in accordance with the invention;

FIGS. 3A and 3B are block diagrams showing still another example of aspectrometer system in accordance with the invention;

FIGS. 4A-4C are schematic views showing sample spectral intensity dataas determined by a detector subsystem in accordance with FIGS. 1-3B atthree different focusing lens positions for a technique used todetermine the optimal focusing lens position in accordance with examplesof the invention;

FIG. 5 is a graph showing the integration of spectral intensity over allwavelengths for ten different focusing lens positions;

FIG. 6A is a graph showing intensity for carbon in a steel sample duringsequential laser pulses in accordance with a cleaning method associatedwith embodiments of the invention;

FIG. 6B is a graph showing intensity for iron during sequential laserpulses in accordance with the cleaning method associated with FIG. 6A;

FIG. 7A is a view of a sample to be cleaned by the LIBS laser of FIGS.1-3B prior to performing an analysis;

FIG. 7B is a view of a portion of the sample of FIG. 7A after cleaning;

FIG. 8 is a flow chart depicting the primary steps associated with amethod in accordance with the invention and/or the programming and/orconfiguration of the controller depicted in FIGS. 1-3B;

FIG. 9 is a flow chart depicting the primary steps associated with thefocusing cycle depicted in FIG. 8;

FIG. 10 is a flow chart depicting the primary steps associated with thecleaning cycle of FIG. 8;

FIG. 11 is a flow chart depicting the primary steps associated with themoving spot cycle shown in FIG. 8;

FIG. 12 is a schematic three dimensional view of a hand held batterypowered LIBS spectrometer device in accordance with an example of theinvention featuring a gas purge subsystem;

FIG. 13 is a schematic view showing a portion of the device of FIG. 12;

FIG. 14 is a block diagram showing the primary components associatedwith an example of a gas purge subsystem;

FIG. 15 is a timing diagram showing a number of laser pulses; and

FIG. 16 is a graph showing the spectrometer signal strength for a numberof purge conditions.

DETAILED DESCRIPTION OF THE INVENTION

Aside from the preferred embodiment or embodiments disclosed below, thisinvention is capable of other embodiments and of being practiced orbeing carried out in various ways. Thus, it is to be understood that theinvention is not limited in its application to the details ofconstruction and the arrangements of components set forth in thefollowing description or illustrated in the drawings. If only oneembodiment is described herein, the claims hereof are not to be limitedto that embodiment. Moreover, the claims hereof are not to be readrestrictively unless there is clear and convincing evidence manifestinga certain exclusion, restriction, or disclaimer.

In the example of FIG. 1, a LIBS laser 10 directs its collimated output,when energized by controller subsystem 12, to adjustable focusing lens14 which produces a small spot (e.g., 100 μm) of laser energy on sample18 creating a plasma. The focusing lens can be moved in the axialdirection, meaning in a direction perpendicular to the surface eithercloser to or further from the sample as shown by arrow 15.

The resulting photons of the plasma produced by the laser energy proceedalong a detection path including focusing lens 14 to subsystem 20 (e.g.,a spectrometer). The output signal of detector subsystem 20 may beprocessed by controller subsystem 12.

In this particular example, high pass filter 21 passes laser energy(e.g., at, for example, 1500 nm) from LIBS laser 10 to lens 14 andreflects lower wavelengths (e.g., below about 1,000 nm) to subsystem 20which may include a slit.

A translation mechanism 22 may be provided under the control ofcontroller subsystem 12 to move focusing lens 14 in the axial directiontowards or away from the sample surface (vertically in the figure) inorder to permit focusing control for rough sample surfaces as well as tocompensate for any path length variations introduced by the optics. Astepper motor combined with gears and the like can be used to adjust theposition of focusing lens 14. An electromagnetic coil or other means oftranslation may also be used.

Spectrometer 20 may include a CCD detector array as set forth in thedesign of co-pending application Ser. Nos. 13/591,907 and 13/507,654incorporated herein by this reference. Other spectrometers includeechelle (with a 2D CCD), Paschen-Runge, and the like.

Controller subsystem 12 may include one or more micro-processors,digital signal processors, analog and/or digital circuitry or similarcomponents, and/or application specific integrated circuit devices andmay be distributed (e.g., one micro-processor can be associated with thedetector subsystem while a micro-controller can be associated with thedevice's electronic circuit board(s). The same is true with respect tothe algorithms, software, firmware, and the like. Various electronicsignal processing and/or conditioning and/or triggering circuitry andchip sets are not depicted in the figures. Additional optics includingbeam expansion, collimation, and/or adjustment optics are possible insome examples. Beam expansion optic 19 is shown for increasing thediameter of the laser output impinging on focusing lens 14. Laser 10 ispreferably a class 1 eye safe laser.

Mechanism 22 may also be configured to move focusing lens 14 right andleft in the figure as shown by arrow 17 (and/or in a direction in andout of the plane of FIG. 1) to move the laser beam spot to multiplelocations on the sample. In one example, controller 12 is configured toautomatically focus the laser beam on the sample, clean the sample,analyze the sample, and then move the laser beam and again properlyfocus the beam, clean the new location, and again analyze the sample.These features are discussed below.

Another way to move the laser beam to multiple locations on the sampleis to use adjustable optic 16, FIG. 2. Optic 16 may include a tip-tiltmirror electromagnetically or electrostatically driven, MEMS mirrors,and the like such as those available from Mirrorcle Technologies, ThorLabs, Newport, as well as other suppliers.

In FIG. 3B, the delivery and return optical paths are similar to thosedescribed for FIG. 1. This example includes a rotating glass window(optic 16) as an alternative method for implementing spot translation onthe sample. The change in refractive index between free space and theglass window combined with the angle of the glass window relative to theoptical axis results in a lateral shift of the laser beam as shown inFIGS. 3A and 3B. Rotating the glass around a glass optic 5 mm thick witha refractive index of 1.5 for example, may be used. If the glass surfaceis angled at 55° to the optical axis, the lateral displacement would beapproximately 2.2 mm. By rotating the glass optic around the opticalaxis as shown in FIGS. 3A and 3B, the focus spot would follow a circleof radius 2.2 mm on the sample surface (e.g., a circle circumference ofapproximately 14 mm). If the glass optic is rotated about the opticalaxis in 6 degree steps, measurements of 60 unique areas of the sampleare enabled each separated by about 0.23 mm.

Another version could include two sequential rotating glass optics,similar to the single optic shown in FIGS. 3A and 3B allowing fulltranslational control in the X and Y directions on the sample ratherthan just being limited to a circle. In still other designs, a compositeglass translation optic could be used to reduce or eliminate refractiveindex dispersion effects which might result in small differences intranslation verses wavelength.

One of the advantages of the geometries of FIGS. 1-3B is that the LIBSlaser and the optical emission detection optics of the detectorsubsystem stay aligned on the same sample point as the sample locationis modified by the movable optic.

Controller subsystem 12 is typically configured (e.g., programmed) toenergize (e.g., pulse) the laser producing a series of laser pulses andto analyze the sample at one location by processing the output of thespectrometer between pulses. The controller subsystem is typicallyconfigured to receive a trigger signal (generated by the operatorpushing a physical or virtual button or the like) and in response topulse the laser. The controller subsystem then adjusts the movable optic(14, FIG. 1; 16, FIGS. 2-3B) and again energizes the laser and analyzesthe sample now at a different location. A typical controller subsystemof a hand held or portable device will typically display, on an outputscreen, the elements detected and, optionally, their concentrations.

Operating the laser in the “eye safe” wavelength range of 1.5 μm offerssignificant advantages to handheld LIBS analyzers. Handheld units are bydesign open beam meaning the laser beam exits the unit before strikingthe sample. Therefore, scattered laser light (or direct laser light inthe case of extreme misuse) could strike the user's eye. However becauselaser light at this wavelength is strongly absorbed by water, the laserlight cannot reach the retina. The laser is therefore rated as low as aClass 1 depending on total energy. A Class 1 rating in particular is asignificant commercial advantage as it eliminates the requirement ofspecial safety glasses be worn during usage and regulatory requirementsare greatly reduced compared to the most regulated Class 4 type oflasers. An eye safe laser may be preferred (e.g., class 1 or 2) and asafer laser source can be used in some embodiments (e.g., class 3) withthe understanding that the class of laser and safe rating depends onvariables such as energy level, wavelength, pulse width, pulse rate,divergence angle, and the like.

However, lasers that operate in the “eye safe” wavelength range near 1.5μm create a number of hurdles, addressed below, that are needed to makethis type of laser practical.

The LIBS technique requires that a burst of laser light strikes asample, and deposits enough heat in the area struck so as to generate aplasma. When the plasma cools, electrons from the various elements thatcomprise the sample fall from various excited states to lower energystates, emitting photons in the process. The frequency of the emittedphoton is proportional to the energy of the photon which is, in turn,equal to the difference between the two energy states. The frequency (orits inverse, wavelength) and intensity of the photons are measured by aspectrometer type detector to determine chemical composition of thesample spot where the plasma was created.

Portable or handheld LIBS systems are designed to operate from batteriesand therefore are limited in power. If a portable or handheld LIBSsystem also uses an eye-safe laser, the energy available in the laser,at least with currently available technology, is further reduced. Inorder to generate a sufficient energy density for plasma ignition in thesample region being analyzed under these conditions, the laser ispreferably focused down to a much smaller spot size than required forhigher power bench top lasers, e.g., on the order of 5 μm-100 μm by lens14, FIGS. 1-3B. The initiation of a plasma is dependent mainly on powerdensity rather than total power. Therefore, a lower power laser must befocused to a smaller spot size to attain sufficient power density forplasma ignition. It is therefore possible to use a much lower poweredlaser that is more conducive to a handheld or portable LIBS unit and yetstill generate a plasma on the sample surface. The main trade-off oflower power lasers is that the ablation area on the sample will bereduced in area resulting in a more localized measurement and a lowersignal.

A small sample area (5 μm-100 μm in diameter) does however createproblems that should be solved to use a portable or handheld LIBS devicefor real-world applications. First, it can be important that the laserbe focused at the location where the analysis is required. For mostsamples, this is the surface of the sample. A small deviation in thefocus position for whatever reason means the laser is focused slightlyabove the sample surface, yielding incomplete plasma formation, or thelaser light strikes the surface before reaching the focal point (whichtheoretically is at some depth inside the sample in this case). Ineither case, an incomplete plasma is formed with poorer light formationor the plasma is not representative of the sample being tested leadingto erroneous analytical results. Also, in many real-world cases, samplesbeing tested are not completely smooth or they are not flat (such aswires, tubes, rods, etc.). In these cases the ideal focus may vary fromsample to sample such as testing a flat piece of steel followed bytesting a ¼″ diameter steel rod or a ⅛″ welding rod or wire. Adjustablefocusing lens 14, FIGS. 1-3B under the control of controller 12 alsoallows for proper focusing in samples with features which block orinterfere with the head of the portable device.

The second issue is sample cleanliness. LIBS is a very sensitivetechnique and the depth of the region being analyzed is typically justseveral microns, coupled with a sample area diameter of 5-100 μm. It istherefore important that the surface being analyzed is representative ofthe sample and is therefore free of dirt, oils and/or oxidation. Priorto taking spectral data to determine composition, it is typical to firea number of “cleaning shots” with the laser. These cleaning shots burnoff material on the surface allowing underlying clean material to beanalyzed. However, as stated above in order for the cleaning tests to beeffective, the laser must be properly focused as well. In batterypowered devices, it is important not to fire cleaning shots which arenot required in order to conserve both battery power and analysis time.

A third issue is sample inhomogeneity. For certain types of samples suchas vacuum melt alloys, the samples are likely very homogeneous over a 50μm-100 μm laser beam spot size. However for geochemical samples (soils,sediments, ores) or liquid suspensions (as a few examples), it is likelythat the concentration of the sample changes over a 5 μm-100 μm samplearea. Therefore, it can be important to fire the laser at severaldifferent locations on the surface of the sample and to average theresults.

In embodiments of the invention, translating mechanism 22, FIGS. 1-3Bmoves the focusing lens 12. At the first scan location, the laser isfired and a spectrum from the sample is acquired. A typical spectrumthat shows intensity of light measured versus wavelength is shown in oneor more of FIGS. 4A-4C. The entire spectrum or one or more regions ofthe spectrum as output by the spectrometer are integrated by thecontroller 12, FIGS. 1-3B. The lens 14 is then moved incrementallythrough a series of positions causing the laser focus to occur in frontof the sample and then progressing into the sample bulk. Intensity datais gathered and stored for each focus position. The lens may be movedfrom a furthest away position to a closest position (a typical range ofabout 6 mm) in 0.01-1 mm increments.

FIG. 4A shows the intensity data where lens 14 is too far away fromsample 18; FIG. 4B shows the intensity data where lens 14 is too closeto sample 18; and FIG. 4C shows the intensity data when the lens 14 isproducing a preferred, optimum spot size (e.g., 50-100 μm) on thesurface of sample 18. In FIG. 4C, the intensity is at a maximum.Controller 12, FIGS. 1-3B, is programmed to detect a maximum or nearmaximum intensity by adjusting the lens focus from outside to inside thesample. The information is then available to the controller on where thelens should be positioned for both large cleaning pulse spots andsmaller spots to be used for data collection.

An example of data from a carbon steel sample is shown in FIG. 5. Theintegrated intensity will approach a peak value at the correct optimalfocal location as shown for position 6 in one or more of FIGS. 4A-4C.For the data shown, the increments were in steps of 600 μm movement forthe focusing lens, although the step size can be made smaller.Therefore, when a sample is placed in front of the device, the firststep is that spectra are gathered at several focusing lens locations inorder to automatically fine tune the focal spot of the laser on thesample. Controller 12, FIGS. 1-3B is configured to perform these stepsas part of the initial testing and to determine and save the focusinglens location that yielded the maximum intensity. After the optimalfocusing position is determined automatically, the processor moves thelens to this location and may then begin testing the sample oroptionally, moves the lens to create a larger than optimum spot size forthe purposes of cleaning, followed by data collection at the optimum(smallest) spot size. The controller is preferably configured to performthis task automatically rather than requiring operator input andjudgment.

Without a process to automatically focus the laser onto the sample, theoperator may not know if the sample results were correct. Theconcentration results determined by the instrument are related to theintensity of light measured in specific regions of the spectrum. If thelaser is not properly focused, the concentration results will beinaccurate. For a commercially viable product, it is desirable that theinstrument automatically determine the correct focusing location for thelaser. Otherwise, an operator would have to manually performmeasurements to make this determination. This may require a far higherskill level operator and therefore could diminish the commercial successof the LIBS device.

A next step in the analysis is to automatically determine if the samplelocation being tested is sufficiently clean. One cleaning cycle methodis to take multiple repeat laser tests of the area and identify two(typically) of the largest spectra (atomic emission) peaks usingavailable peak finding algorithms. Smaller peaks may also be selectedthat are important to the analysis at hand. These peaks correspond toparticular elements present in the sample area being tested. Additionallaser tests of the sample area are performed. Controller 12 thencomputes a rolling average of the intensity measured for the above twoelements. When the intensity stops changing by less than a predeterminedpercentage from each point in the rolling average (for example by lessthan 5%), then the sample is appropriately cleaned. An alternativemethod for determination of cleanliness would be to compute theintensity ratios of the rolling averages. Once the ratio stabilizes towithin a preset percentage, the sample may be considered to be clean.

An example of peak intensity verses cleaning pulse count is shown inFIGS. 6A and 6B for a sample of rusty carbon steel (photo in FIGS.7A-7B). The cleaning cycle requires that a layer of dirt and oxidationbe burned off by the laser blasts. As shown, the intensity of the carbonpeak (FIG. 6A) and iron peak (FIG. 6B) change with sequential tests(laser pulses) until the results approach a stable intensity level.Here, 25 laser pulses resulted in a stabilized detector output. Themethod in this case may use a five point moving average. The carbonintensity (FIG. 6A) decreases with the sequential cleaning tests ascarbon-containing organic material (i.e. dirt, oils or skin oils) areburned of the sample. The point at which the carbon intensities stopdecreasing indicate that only the base metal is being tested.

Likewise, the iron concentration (FIG. 6B) increases during the earlycleaning tests as oxidation and other materials are burned off whichwere masking the iron content in the sample. Again, as the change inintensity of the iron photon emissions flatten with increasing testnumber, the base iron in the sample is being analyzed.

In principle, it may possible to only use a single peak for theautomatic determination of when to end the cleaning tests. In addition,when testing for low concentrations of an element, say 1% carbon in 99%iron, the carbon line will be far more sensitive to cleanliness than theiron since the ratio of contamination to sample carbon is large and theratio of contamination to iron is small. The peaks which are selectedfor analysis may include typical elements in the bulk sample or in thecontamination coating such as carbon, oxygen, and silicon. Byautomatically stopping the cleaning cycle when the sample issufficiently clean, battery power is conserved and testing time isreduced.

It should be noted that the process of finding the optimal focal lengthfor the sample, described above, also provides some cleaning of thesample spot, thereby reducing the number of cleaning tests performed inthis step.

One preferred cleaning method also results in an optimal manner toperform the cleaning and the subsequent sample analysis. Based on thetesting performed to develop this method, a number of observations weremade about the sample cleaning. Consider the pictures of a sample shownin FIGS. 7A and 7B. Upon examination of the area struck by laser, it wasobserved that the inner portion of the circular laser spot area 40, FIG.7B, is well-cleaned but near the perimeter 42 of the analysis area, thecleaning may be less thorough.

In the real world of non-ideal lenses, lasers, and diffraction limitedoptics, it is expected that the inner component of the laser beam willdeliver more energy to the sample than the outer perimeter of the beam.The region of the sample will thus be better cleaned more towards thecenter of the sample area. Therefore, an additional embodiment of thecleaning cycle method is to clean a larger area, in one example, than isactually analyzed. After the controller determines the optimal laserbeam focal length as described previously, the focusing lens is movedsuch that the beam striking the sample surface during cleaning tests isabout 20% larger. See, e.g., FIG. 4B. When the controller determinesthat the sample region is adequately cleaned, according to the abovedescribed steps, then the controller returns the focusing lens to theoptimal position previously determined and stored. This assures that thearea struck by the laser during analysis is therefore smaller than thearea cleaned assuring that the area to be analyzed is thoroughlycleaned.

Another problem addressed is sample non-homogeneity. Many samples, forexample geochemical samples encountered in the analysis of soil, ores,sediments and/or slurries are not homogeneous across the sample face. Inother techniques, such as x-ray fluorescence analysis, the samples arecollected and ground to about a 100 μm particle size prior to analysis.However, 100 μm is approximately the same size as the laser beam on thesample in the case of a LIBS analysis in accordance with embodiments ofthe invention. It is therefore desirable to test multiple locations onthe sample and average the results.

The method provides for an optical/mechanical means which moves thelaser beam spot across the sample as discussed with respect to FIGS.1-3B to address the problem of non-homogeneous samples. FIGS. 3A-3B, forexample, show an optical component 16 that is angled with respect to thelaser beam striking it. As the optical component rotates by discreteamounts, the laser beam is directed to different locations on thesample. Therefore, a preferred method used locates the laser beam at aparticular spot on the sample, finds the correct focal length bytranslating the focusing lens 14, and then performs the cleaningoperations as described above, followed by the sample analysis. Theoptical component 16 is then rotated a discrete amount, for example 60degrees, to yield a different sample location which it is also cleanedand analyzed. At each location, the optimal focus is determined andsaved for the laser spot on the sample as described above. In a furtherembodiment, if the analytical results (e.g., the concentration of thetop five elements changes by 10% or less) for a second or third testinglocation are not appreciably different than the first testing location,the controller terminates the measurement process and the controlleraverages the results. Thus, for homogeneous samples, only two to threelocations are cleaned and analyzed conserving power in a batteryoperated device. For non-homogeneous sample, five locations may becleaned and analyzed. The controller is preferably configured to reportto the operator when the sample is homogeneous and/or non-homogeneous.Note that XRF techniques are not able to determine if a sample isnon-homogeneous.

FIG. 8 depicts the processing of controller 12, FIGS. 1-3B in onepreferred embodiment. The focusing cycle is initiated, step 60, inresponse to a trigger signal followed by the cleaning cycle, step 62,for each sample location. These cycles may be reversed. At each locationon the sample, the spectrum analysis is performed, step 64, wherein theelemental concentrations are computed, reported, and typically saved.The hand held portable unit, see FIG. 12, preferably has a displayscreen for displaying the elements present in the sample, theirconcentrations, and other data. In general, the controller subsystem isconfigured, (e.g., programmed) to pulse the laser producing a series oflaser pulses and to process the resulting signals from the detector(spectrometer) subsystem to determine one or more elementalconcentrations in the sample. For LIBS analysis, the detector outputssignals representing intensities at different wavelengths defining theelements in the sample and the various concentrations.

The laser beam spot is then moved, step 66 whereupon the focusing,cleaning, and analysis cycles repeat for the new sample location.Sequential locations are thus analyzed.

In the focusing cycle, the controller is configured to adjust thefocusing lens, step 70, pulse the laser, step 72, and analyze theintensity data reported by the detector electronics, step 74 (see FIGS.4A-4C) until an optimum intensity is detected (which is at or near themaximum), step 76. The lens position which resulted in the optimumintensity is stored, step 78. A memory accessed by the controller may beused to store lens position values, calibration constants, spectraldata, algorithms, computer code, and the like. FIG. 5 demonstrates anoptimum focus location in the range of positions 4 to 7.

In the cleaning cycle, FIG. 10, the focusing lens is moved to theoptimal position, step 80, or optionally moved to produce a slightlylarger spot size, step 82. The laser is repeatedly pulsed, step 84, andfor each resulting plasma, one or more peaks are analyzed, step 86. Thecleaning cycle stops when the intensity data indicates the intensity hasstabilized, step 88 and 90 as shown in the example of FIGS. 6A and 6B.

The moving spot cycle, FIG. 11, preferably includes running the focusingcycle, step 60 and running the cleaning cycle 62 at each location on thesample. At the optimal laser spot size, the laser is pulsed, step 92 andthe spectrum is analyzed, step 64. Typically, a minimum number of samplelocations are tested (e.g., 3), step 60 as depicted at 94 and if theminimum number has not been reached, the movable optic (14, FIGS. 1-3B,16) is adjusted to move the beam to a different location on the sample,step 96, FIG. 11. The focusing, cleaning, and analysis cycles are againrepeated for this new sample location until the analysis as betweendifferent sample locations indicates a homogeneous sample as shown at 98or a maximum number of sample locations (e.g., 5) have been tested, step100 (for non-homogeneous samples). Alternatively, once the sample isdetermined to be non-homogeneous, a predetermined number of new samplelocations may be analyzed. Preferably, the results are averaged for bothhomogeneous and non-homogeneous samples and reported, step 102.

The number of required sequential sampling locations may depend on howheterogeneous the sample is. It is desirable to minimize the requiredsampling time, so various algorithms may be employed as data iscollected to optimize the sampling time required. One algorithm startswith a minimum sampling location count (3 locations for example) toestablish a baseline variance or standard deviation in constituentconcentration. If the standard deviation is above a pre-set threshold,then the algorithm will initiate further measurements from additionalsample N locations.

Each time a new location is sampled, the standard deviation of the dataset is calculated. The precision of the mean (or average) concentrationis related to the standard deviation and the number of samples N in thedata set by:

$\begin{matrix}{\sigma_{mean} = {\frac{\sigma}{\sqrt{N}}.}} & (1)\end{matrix}$

The “standard deviation of the mean” is a measure of how stable thecomputed average of the measured concentrations are. The algorithmterminates further sample location measurements once the standarddeviation of the mean is below a pre-set threshold. Often with suchalgorithms, a maximum sample location count is programmed to force theinstrument to stop measuring after a certain time limit is reached. Suchalgorithms can also make estimates of time to completion based on therate of improvement of the “standard deviation of the mean” (orsimilarly computed indicator) after the first several measurements. Theuser may be given the option to wait for completion or to stop themeasurement.

FIG. 12 shows a handheld portable unit housing the subsystems andcomponents of FIGS. 1, 2, and/or 3B and with the associated electroniccircuitry carrying out the analysis, signal processing, and controlsteps depicted above with respect to FIGS. 4A-6B and FIGS. 8-11.

An argon purge subsystem may be included for better analysis of thesample for certain elements including sulfur, phosphorous, and/orcarbon.

In some embodiments, the focusing lens adjustment cycle is performedwithout moving the laser spot to multiple locations on the sample andvice versa. The cleaning cycle is, in some embodiments, preferred and inanother aspect is optional and/or separately patentable.

In one preferred embodiment, the hand held LIBS spectrometer is batterypowered and employs an eye safe laser. The automatic focusing stepsensure repeatable, more accurate elemental concentration results withoutoperator intervention. Automatic focusing provides more repeatableresults, without operator intervention, and more accurate results.

The cleaning cycle ensures that the laser adequately cleans the samplewhile at the same time saves testing time and battery power because,once the sample is adequately cleaned, no more cleaning laser pulses areneeded. This reduces the number of laser shots and therefore makes thetest conclude faster and saves battery power.

Adequate sampling of all samples is performed and battery power andtesting time are conserved.

FIG. 12 shows one example of a battery powered, portable LIBS analyzer200 with gas (e.g., argon) cartridge 202 loadable therein. As shown inFIG. 13, one or more nozzles 204 a is fluidly connected to cartridge 202via valve 206, FIG. 12. In other examples, a cartridge or small tank isconnected to unit 200 and carried in a small pack for field analysis.

Preferably, only a small supply of argon is required in the purgesubsystem because the nozzle(s) is configured to deliver a small sprayof argon gas locally in a small purge volume. Unit 200 may have aconverging front nose 210 where the laser beam exits to strike sample212 at location 214 (e.g., 5-100 μm in diameter) creating a plasma 219.Nozzle 204 a is just inside distal nose 210 proximate end wall 216 andoriented to produce an argon spray at (and preferably only at) location214. The nozzle has an orifice configured to produce a purge volume ofargon gas less than 1.0 cm³, typically as small as 0.5 cm³ as show at218 so it just surrounds the plasma 219 and little argon is wasted. Inone example, the argon gas volume was 0.125 cm³. As discussed below, theflow rate is low and the argon purge is used only when needed in orderto further save argon resulting in a WS analysis unit which does notrequire a large supply of inert gas.

FIG. 14 shows controller 12 controlling solenoid valve 201 betweensource 202 and nozzle 204 a. A trigger signal as shown at 220(generated, for example, by pressing on trigger mechanism 222, FIG. 12)is received at controller 12 and, in response, controller 12 mayoptionally initiate the cleaning cycle as discussed above. Anothertrigger mechanism may include a physical or virtual button.

During the subsequent analysis cycle, controller 12 opens valve 206 justprior (e.g., 0.1-0.5 seconds before) the first plasma producing laserpulse as shown in FIG. 15. FIG. 16 shows a strong signal for carbon in atest sample even when the purge occurred just 0.1 seconds prior to thefirst laser pulse. Controller 12, FIG. 14 is further configured to closevalve 206 shortly after the last laser pulse or even prior to the lastlaser pulse as shown in FIG. 15 in order to conserve the purging gas.

FIG. 16 depicts the influence of the flow rate, nozzle position, andpurge timing on the resulting LIBS signal output by spectrometer 20,FIG. 14. In test A, no purge was used and the carbon peak was difficultto correctly decipher. In test B, the purge rate was 4 CFH, the nozzlewas 0.2 cm away from the sample and the plasma location, and the purgegas was initiated 0.5 seconds before the first laser pulse. In test C,the nozzle position and the flow rate were the same as in test B but nowthe purge gas was initiated only 0.1 second before the first laserpulse. The signal strength was still very high. In test D, a lower flowrate of 0.5 CFH was used and the purge occurred 0.5 seconds before thefirst laser pulse while in test E the lower gas flow rate of 0.5 CFH wasused and the solenoid valve was opened for a purge 0.1 seconds beforethe first laser pulse. In both cases, the signal strength wassufficiently high. In test F and G the nozzle was brought closer to thesample (0.1 cm away from the sample). In test F a flow rate of 4 CFH wasused with a 0.5 second purge delay and in test G a 0.5 CFH flow rate wasused with a 0.5 second purge delay.

Accordingly, it is possible to use a very low flow rate of 0.5 CFH and avery short (0.1 second) delay before the first laser pulse and stillobtain a sufficiently strong signal from the resulting photons. A purgerate of less than 2 CFH may be optimal.

In one typical scenario, the output of spectrometer 20 is analyzedbetween the laser pulses shown in FIG. 15. In some examples, if thevalve can be actuated at a high frequency rate, the gas can even beturned off between laser pulses and then on again just prior to eachlaser pulse.

Thus, in one preferred embodiment, an improved signal is generated anddetected by the spectrometer using an inert gas purge. The gas isconserved by using a low flow rate and a smaller size nozzle properlylocated and oriented to produce a small volume purge spray. And, thepurge is used only when required. One result is the ability to use onlya small cartridge as opposed to an unwieldy tank in a portable, handheld, battery powered system. When one cartridge is emptied, anotherfull cartridge can be quickly loaded into the unit.

Other embodiments will occur to those skilled in the art and are withinthe following claims. One example includes a Raman laser and Ramanspectroscopy.

What is claimed is:
 1. A battery powered, portable LIBS analyzercomprising: a laser source generating a laser beam received by anadjustable focusing lens for creating a plasma at a location on asample; a spectrometer responsive to photons emitted by the sample atsaid location and having an output; a cartridge of inert gas; a nozzleconfigured to deliver inert gas from the cartridge locally to saidlocation on the sample; a valve between the cartridge of inert gas andthe nozzle; a trigger mechanism generating a trigger signal; and acontroller, responsive to the trigger signal, and configured to:initiate a focusing cycle by pulsing the laser while adjusting thefocusing lens based on said spectrometer output to focus the laser beamat one location on the sample, initiate a cleaning cycle to clean saidlocation by pulsing said laser for a number of pulses based on theoutput of said spectrometer, initiate an analysis cycle by pulsing saidlaser while controlling said valve to purge said location with inert gasand analyzing said spectrometer output, move the laser beam to a newlocation on the sample, and repeat said cleaning cycle and said analysiscycle at said new location.
 2. The analyzer of claim 1 in which saidcartridge is loadable with the analyzer.
 3. A method of operating abattery powered, portable LIBS analyzer comprising: initiating afocusing cycle to adjust the focus of a laser beam on a sample at alocation based on an output of a spectrometer; initiating a cleaningcycle to clean said location by pulsing the laser for a number of pulsesbased on the spectrometer output indicating said location issufficiently clean; initiating an analysis cycle to analyze saidlocation by pulsing said laser while purging said location with an inertgas and analyzing said spectrometer output; moving said laser beam to anew location on the sample; and repeating the cleaning cycle and theanalysis cycle at said new location.